Molecular phylogeography of the troglobiotic millipede Tetracion Hoffman, 1956 (Diplopoda, Callipodida, Abacionidae)

Abstract

More than 85 species of cave-obligate (troglobiotic) millipede have been described from North America. Understanding the patterns and processes that determine their distribution in this region is an area of recent research. Here, we present the first molecular phylogeographic study of troglobiotic millipedes. Millipedes of the genus Tetracion Hoffman, 1956 (Callipodida: Abacionidae) inhabit caves on the Cumberland Plateau in Tennessee and Alabama, a global hotspot for cave biodiversity. Three species have been described: T. jonesi Hoffman, 1956, T. antraeum Hoffman, 1956, and T. tennesseensis Causey, 1959. To examine genetic divergence within and between species of Tetracion we sequenced part of the mitochondrial cytochrome oxidase 1 gene from 53 individuals from eleven caves across the range of T. tennesseensis and in the northern part of the range of T. jonesi. We found: (1) little variation within species (six haplotypes in T. tennesseensis and four haplotypes in T. jonesi, with a maximum of 1.4% intraspecific divergence between haplotypes), (2) that gene flow between caves is limited (7 of 10 haplotypes were restricted to a single cave, and F ST > 0.80 and P < 0.05 for fifteen of eighteen comparisons between caves), and (3) significant genetic divergence between species (8.8% between T. tennesseensis and T. jonesi). Our results are consistent with previous morphology-based species definitions showing T. tennesseensis and T. jonesi belonging to distinct taxa. Our research contributes to the growing body of phylogeographic information about cave species on the Cumberland Plateau, and provides a point of comparison for future studies of troglobionts and millipedes.

Morphometrics and phylogeography of the cave-obligate land snail Helicodiscus barri (Gastropoda, Stylommatophora, Helicodiscidae)

“…Recent cave bioinventory efforts within the ILP and Appalachians karst regions have yielded several additional specimens of this species for comparison across multiple physiographic provinces. The disjunct, mosaic distribution pattern of H. barri in conjunction with a lack of clear morphological variation is consistent with a high potential for cryptic diversity, as observed in other subterranean taxa (Snowman et al 2010, Loria et al 2011, Niemiller et al 2012, Inäbnit et al 2019. We examined museum accessions of H. barri while sampling caves within the states of Tennessee, Alabama, and Georgia for additional specimens.…”

“…This may promote isolation and short-range endemism in troglobites (Culver et al 2009, Niemiller andZigler 2013). No phylogeographic study of troglobitic snails has been conducted in North America, and all other molecular studies of troglobites in the Appalachians and Interior Low Plateau have focused on organisms with comparatively higher vagility and dispersal potential (e.g., Buhay et al 2007, Niemiller et al 2008, Niemiller et al 2012, Snowman et al 2010, Loria et al 2011. Using both a multilocus molecular and a morphometrics approach, we investigated genetic diversity within H. barri to identify potential cryptic populations within the species' range, and to further determine whether external shell morphology was a useful indicator of differing patterns of genetic variation.…”

“…Mitochondrial genetic divergence among H. barri populations is significantly higher (16.14%) compared to other troglobitic invertebrate taxa studied in the region (e.g. 3.1% for Nesticus spiders (Snowman et al 2010); 0.06% for Tetracion millipedes (Loria et al 2011);2.1% for Ptomaphagus beetles (Leray et al 2019)), suggesting that the low vagility of land snails accentuates the isolation caused by subsurface habitat fragmentation. Rates of mitochondrial gene evolution for land snails vary considerably, with estimates of 1.6-12.9% per million years for ribosomal genes and 2.8-13% for CO1 (Thomaz et al 1996, Chiba 1999, Van Riel et al 2005.…”

Is geography an accurate predictor of evolutionary history in the millipede family Xystodesmidae?

“…Perhaps especially in groups with low dispersal capabilities like xystodesmid millipedes. For example, studies implementing Templeton-Crandall-Sing parsimony and inferring demographic processes using nested clade analysis in millipedes, have generated hypotheses useful for conservation and understanding phylogeographic patterns ( Zigler, Loria & Lewis, 2011 ; Marek & Bond, 2009 ). However, in very old groups of non-mobile invertebrates, the technique of using geographical proximity is more challenging ( Hedin & McCormack, 2017 ; Hendrixson & Bond, 2007 ).…”

A remarkable new species of the millipede genus Trachyjulus Peters, 1864 (Diplopoda, Spirostreptida, Cambalopsidae) from Thailand, based both on morphological and molecular evidence

“…This may be accounted for by the much smaller sizes of Trachyjulus spp., as well as their usually more limited dispersal capacities that make them largely restricted to a particular cave or cave complex (Golovatch et al 2007a(Golovatch et al , 2011a. High rates of interspecific genetic differentiation in small-bodied cave-dwelling species have long been reported elsewhere: 8.2-9.2% between two parapatric Callipodida millipedes from the USA, Tetracion tennesseensis Causey, 1959and T. jonesi Hoffman, 1956(Loria et al 2011.…”